1. Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over digital communications networks and, more specifically, to methods and apparatus for precision time transfer over optical fiber.
2. Description of the Related Art
It has been recognized that synchronizing network elements in optical communications networks to a high level of precision enables the provision of advanced services. Consequently, time and frequency alignment are essential to certain types of systems operating in conventional optical networks. For example, accurate time alignment is required by instrumentation systems gathering data at specific time intervals, services carried out in real time over a network, and network elements that use packet-based signal formats for multiplexing, transmission, and switching. Similarly, frequency alignment is required in time-division multiplexing (TDM) and media streaming systems that require fixed video or audio sample rates across multiple clients.
One approach known in the art that provides both time and frequency alignment involves computing an aligned time signal based on a master timing signal from a primary reference clock, such as a global positioning system (GPS) satellite timing signal, which is held in precise alignment with a global clock reference. Using GPS signals or other master timing signals at each network element to achieve time or frequency alignment is generally prohibitively expensive and requires each network element to be able to receive satellite time signals from GPS satellites. In addition, there are many situations where visibility of GPS satellites may be compromised or interrupted. Consequently, a more cost-effective approach to time alignment is to transmit timing alignment information via a protocol that is operable within a given communications network.
In conventional TDM networks a physical layer method implements frequency alignment throughout the network, starting with a designated master clock system. The designated master clock system delivers frequency and/or timing information via bit-timing and/or symbol-timing information associated with downstream physical communication links. In normal operation, each network element coupled to the master clock system regenerates and distributes the master clock timing information to neighboring downstream network elements in a point-to-point fashion over the physical medium interconnecting adjacent network elements. Thus, each network element within the TDM network receives frequency and/or timing information and aligns local frequency and/or timing with an upstream clock reference, thereby enabling every network element within the TDM network to achieve frequency alignment. Provided that adequate care is taken to avoid timing loops, such a configuration has been proven to be robust. However, the timing reference transferred between elements in the TDM environment is principally'a frequency reference as opposed to a time reference.
Packet-based methods such as Precision Time Protocol (PTP) and Network Time Protocol (NTP) transfer time and frequency references using packets containing time stamps that identify the times of departure/arrival of packets. PTP and NTP can distribute timing and frequency alignment throughout a network in a point-to-point fashion similar to the way that TDM networks distribute frequency alignment, as described above. PTP and NTP can also operate in a mode where the “slave” clock in a network element can communicate directly with the “master” clock system for timing purposes. In either case, the accuracy of such two-way time-transfer protocols is adversely affected by packet delay variation introduced by the intervening network elements and optical links. PTP and NTP assume that transit delays between master and slave clocks are symmetric, i.e., the transfer packet delay from a master clock to a slave clock is equal to the transfer packet delay from the slave clock to the master clock. But because of the fundamental statistical behavior of packet networks, the transit delays are not fixed and can vary from packet to packet or with direction of data transmission, i.e., master-to-slave transmissions versus slave-to-master transmissions. Specifically, the asymmetry in transit delay of timing packets between slave and master clocks provides a statistical bound to the accuracy of time transfer.
Also, packet-based methods like PTP and NTP often use separate fiber strands and/or fiber wavelengths for carrying signals in each direction, i.e., slave-to-master and master-to-slave, and assembly and deployment methods of communication networks often include short lengths of cable for mounting convenience that vary each path length between network elements by an unknown amount. The asymmetry in transit delay resulting from such variation in path length can be tens of nanoseconds or more, while the desired level of time accuracy and time stability in a fiber-optic communication network can be on the order of nanoseconds. Thus, the ability of PTP and NTP to accurately transfer time between network elements in a fiber-optic network is limited.